Re: [PATCH] KVM: nVMX: nested TPR shadow/threshold emulation

[Paolo Bonzini]

Il 01/08/2014 02:57, Zhang, Yang Z ha scritto:
> > TPR_THRESHOLD will be likely written as zero, but the processor will
> > never use it anyway.  It's just a small optimization because
> > nested_cpu_has(vmcs12, CPU_BASED_TPR_SHADOW) will almost always be true.
> 
> Theoretically, you are right. But we should not expect all VMMs
> follow it. It is not worth to violate the SDM just for saving two or
> three instructions' cost.

Yes, you do need an "if (cpu_has_vmx_tpr_shadow())" around the
vmcs_write32.  But still, checking nested_cpu_has is not strictly
necessary.  Right now they both are a single AND, but I have plans to
change all of the cpu_has_*() checks to static keys.

Paolo
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/